Age hardening alloy steel of high hardenability and toughness



United States Patent Ofihce 2,715,576 Patented Aug. 16, 1955 AGE HARDENING ALLOY STEEL OF HIGH HARDENABILITY AND TOUGHNESS Peter Payson, New York, and William George Johnson, Bronxville, N. Y., assignors to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey N Drawing. Application April 21, 1954, Serial No. 424,757

15 Claims. (Cl. 75-124) This invention pertains to age or precipitation hardening alloy steels and to articles thereof.

It is an object of this invention to provide a relatively low alloy, age hardening steel, that is hardenable to an appreciable degree, that is, to upward of Rockwell C 35, by a relatively low temperature precipitation hardening treatment that is, around 900 to 1000 F. It is further the object of this invention to provide such a steel which as so hardened, has a relatively good notch impact value, that is, over about 10 ft. lbs. as measured by the conventional V-notch Izod test piece. Another object of this invention is to provide articles of .such steel as so heat treated for use as die-blocks for die-castings or forgings of zinc and aluminum alloys; and for structural parts, such as shafts, gears, connecting rods, and so on, which are required to have high strength and toughness in large sections.

In the production of large parts of high strength, that is, those having sections over about 2 inches thick, it is necessary to use steels of high hardenability. Heretofore steels employed for such purposes are those which are hardenable by a quench and temper treatment, and the practice has been to heat treat parts made thereof by a quench and temper treatment after they have been machined. Such heat treatment, no matter how carefully carried out, introduces some size change in the steel and parts made thereof. Size change is of course undesirable in closely fitting parts and when it occurs a considerable amount of labor has to be expended in grinding and polishing parts after they are heat treated to restore the desired precise dimensions to them. Furthermore, when parts are very large, say over 10 to 12 inches in section there are few if any steels available which will develop high strength with good toughness by means of the conventional quench and temper procedure.

The steel of our invention has the advantage over conventional steels in respect both to size-change in hardening and limitation of size for effective hardening, in that it does not depend for its development of hardness on the formation of austenite and the subsequent transformation of austenite to the hard transformation products, martensite and bainite, with the unavoidable accompanying increase in volume of the steel. The steel of our invention hardens while it is held at a temperature of about 900 to 1000 F. and undergoes practically no size change during such hardening. Furthermore, the steel of our invention hardens throughout uniformly regardless of section since the hardening takes place during the heating and is independent of cooling rates, in contrast to the hardening of conventional steels in which hardening takes place during cooling from an austenitizing temperature, and in which different cooling rates, which vary of necessity from the outside to the center of large sections, bring about different degrees of hardening.

The steel of our invention contains nickel in amounts between 2.5 and 6% and aluminum in amounts between 0.8 and 3%, and preferably also molybdenum from about 0.10% up to about 2.0%. As is well known, nickel and aluminum form a compound which is soluble in ferrite at about 1200 to 1300 1 and which can be made to precipitate subsequently by reheating the steel for fairly long times at about 900 to 1000 F. Since the aging temperature coincides with the temperature which is known to develop undesirable temper brittleness in steels, and since molybdenum minimizes temper brittleness, it is desirable to have a minimum of about 0.10% molybdenum in the steel, and preferably larger amounts up to about 2%. Molybdenum is desirable also for improving hardenability of the steel for the treatment of large sections prior to aging as will be discussed later.

Since the hardening of the steel of this invention does not depend on martensite formation, it is not essential that the steel contain sizeable amounts of carbon to develop hardness over about Rockwell C 35 and as shown hereinafter, the carbon content should not exceed about 0.55% if the desired notch toughness in the hardened condition is to be obtained. However, it is desirable for carbon to be present in the steel of this invention in amounts from about 0.10 up to about 0.55% in order to make the manufacture of the steel easier in regard to selection of raw materials; to improve the soundness and cleanliness of the finished steel; and to improve the hardenability of the steel for the prior treatment as will be explained later.

Manganese and silicon are present in the steel for desulfurization and deoxidation purposes. Manganese is also effective for improving the hardenability of the steel for the prior treatment as will be explained later and the manganese content may be up to 2%. However, to avoid difliculty in fabricating the steel, the silicon content should not exceed about 1%. Chromium also may be present in the steel up to about 2% without detriment to the hardening and toughness characteristics of the steel. It is also desirable for improving the hardenability of the steel for the prior treatment, as will be explained later.

The steel as outlined above as ordinarily produced has a definite shortcoming, and that is, a lack of notch toughness in the hardened condition. Now we have discovered, and this is the crux of our invention, that by additions of at least one of the active carbide formers, vanadium or columbium (or tantalum), or titanium, we can improve the notch impact resistance of the hardened steel appreciably. Furthermore, the addition of the carbide forming element must be apportioned within critical limits to the carbon content of the steel. Thus a vanadium addition should be limited to from a percentage equal to three-tenths the carbon content to a percentage equal to four times the carbon content; a columbium addition should be limited to from a percentage equal to four-tenths the carbon content to a percentage equal to six times the carbon content; a tantalum addition should be limited to form a percentage equal to eight-tenths the carbon content to a percentage equal to eleven times the carbon content; and a titanium addition should be limited to from a percentage equal to two-tenths the carbon content to a percentage equal to three times the carbon content.

Where more than one of the aforesaid active carbide forming elements is incorporated in the steel of the inven tion, the minimum carbon content should be at least equal to one-fourth of the vanadium content plus one-sixth of the columbium content plus one-eleventh of the tantalum content plus one-third of the titanium content.

Aside from the elements above mentioned the steel of the invention may contain up to about 5 percent in aggregate of other elements which do not impair the hardening and toughness properties of the steel, such as, for example, additions of cobalt, copper, tungsten, etc., the balance of the steel being substantially all iron.

As has already been indicated, the hardening of the steel of this invention is carried out by an aging at 900 to 1000 F. for durations up to about 15 to 20 hours. Before the steel is hardened, however, it must be put into which precipitates during the aging treatment.

a-- condition suitable for machining. The treatment to make the steel machinable lShOUld also include a step which constitutes a solution treatment for the compound This treatrnent pri'or to aging will hereinafter be referred was the prior treatment.

The solution temperature for the age hardening compound "is above about 1200" F. The prior treatment must therefore include-a heating at'a temperature of 1200 'F.

or higher followed by a relatively rapid cool such as an air cool or an oil 'quench. This prior treatment may comprise any one of three procedures as follows:

"(I') The as cast, as rolled, or as forged, steel may be merely heated to a temperature of 1200 F., or higher, and air cooled or oil quenched. If the steel is to'be oil quenched the heating temperature must be below about 1350" Fr, otherwise the steel will be hardened as quenched, since the A-l temperature of the steel is about 1300 to 1350 'F.', depending on the composition. Such martensitic hardening is undesirable from the viewpoint of ma- 2 chin'ability'of the steel; (I'I-)'The as cast, as rolled, or as forged, steel maybe heated to a-temperature above the A-l temperature of the steel, say between 1350"and 1700 F., and cooled slowly'down to about-1200 "F.,' and then cooled in air to room temperature. This constitutes an annealing treatment, but since the steel in this treatment has been held at temperatures over 1200 F. it also constitutes a solution treatment for the age hardening compound.

Of the three prior treatments outlined above, treatment (III), namely, the oil quench from above A-3 followed by the temper at 1200to 1350 F. is preferred, because the structure produced by such treatment, namely, a tempered martensite or other low temperature transformation product, ismuch more conducive to good toughness in the hardened steel than one containing free ferrite. Prior 7 treatments ,(I) and (II) are much more likely .to result in structures containing free ferrite than is prior treatment That a tempered martensite structure is preferred to one containing free ferrite is established by the following:

Two samples of a bar which analysed percentagewise as follows,

0 Mn Si Ni Or V M0 Al w'ereheated to 1650 F.,' and then one was oil quenched and the other was cooled in the furnace to 1300" F., and then at a rate of 25 degrees per hour to 1200 F. and finally'air cooled. The oil quenched piece was tempered at 1300" -F. for four hours. Both pieces were then aged at 900 F. for 16 hours. The age hardened pieces were machined to standard V-notch Izod pieces and broken at room temperature. The data obtained are as follows:

Hardness Itpck- V V I V 7 well 0 g Treatment Structure Z0 Prior Alter Agfi to ing 16 hrs. gl 9 Aging at 900 F.

quenched from 1,650 F and Tempered Martensite- 32.5 41 15 17 tempered 1,300 F. for 4 hrs. Annealed 1,650 F. and slow Primarily free ferrite 25 41 7-8 cooled to 1,200 F. with less than 25% intermediate product.

a as cast as'rolled, -or as forged, steel may be heated to a temperature over the A-'-3 temperature 'oft-he steel say 1500 to 1700 F. and quenched in oil, and then tempered at about- 1200 to .1350 E2, and air cooled. By-such a treatment the steel simultaneously gets a grain refinement-treatment, because it has been heated above theA- temperature, and also develops a structure relatively free. from ferrite, sinceduring the quench the austenite' formed above -A-3 transformsto martensite, or

some otherlow temperature transformation product; In the heatingito 1200? to 1350' subsequent'to the-quench, themartensite or other low temperature transformation product is tempered so'that-the steel is made satisfactorily machinable, and at the same time the age hardening compound is'put into solution.

This shows clearly that for a given composition, the preferred prior treatment is one which results in a tempered martensite structure. It should also be clear now that the steel should have high hardenability so that large sections after the prior treatment should have structures consisting of tempered martensite with a minimum of free ferrite.

After it is hardened by aging as described above, the steel .made without the proper addition of V, Cb, Ta, or Ti generally has a low notch-impact value, around 5 ft. lbs., 7 as measured by the V-notch Izod test. However, when the steel is .made with proper additionsof the active carbideforming elements, the notch-impact is appreciably improved and is generally over 10 ft. lbs. V-notch :Izod, as is shown in Table I.

Table I. V-Notch Izod values of age hardening steels [Treatment prior to aging cgigsisted of an oil quench from 1,650 F. and a 4 hour temper 1,300 F. Aged for '16 hours at QOO F.]

Norn.-"-fhese steels ako'eonttiin-Mn'fromllfi to 1.4%;1? from .O07to 035%; *5 from mo to .040%; S horn 0,2 159.03%; balance substantially all iron.

2,715,576. 5 The data in the above tabulation show that impact values over ft. lbs. V-notch Izod can be obtained in this steel as age hardened to over Rockwell C 35. However, as steel of the active carbide forming elements V, Cb (or has already been indicated, the desired notch-impact values Ta), or Ti, it is our opinion that the following is reasonare not dependent merely on the presence of the active 5 a l carbide-forming elements in the steel, but on the balance 1. In the absence of the active carbide forming elebetween carbon and the carbide-forming element, as may Inents (other than molybdenum and its recognised equivbe seen in the tabulations below. alent, tungsten) all of the carbon dissolves in the austenite during the quench-temper prior treatment and the result- 1 ant tempered martensite is relatively high in carbon.

2. When the active carbide formers V, Cb (or Ta), or Ti are present in the steel in moderate amounts, some of the carbon in the steel is combined with them in the form tion to account for the improvement in the notch-impact of the hardened steel brought about by the addition to the Table II.-Efiect of balance of carbon and vanadium on notch impact value of age hardened steel [Treatment prior to aging consisted of an oil quench from 1,650 F. and a 4 hour temper at 1,300 F. Aged for 16 hours at 900 F.]

! lysis Percent of insoluble carbides. During the quench and temper B Ratio Hardness Vmtch prior treatment, only that portion of the carbon which has at v to C Rockwell Izod,

"c" ram not combined with the active carblde formers dissolves 0 N1 M0 Al V 1n the austemte and the resultant tempered martenslte 14 L2 53 40 3 is relatively low 1n carbon. The steel WIth the relatlvely 2612--- 22 3.7 .67 1. g .22 2. g 115 low carbon tempered martenslte has higher notch-impact 2206--. 21 3.6 .64 1. 1. 28 as L0 L07 as 41 15 after aglng than the steel with relatively high carbon tempered martens1te.

3. The introduction of an excessive amount of the active carbide formers causes all of the carbon in the steel to combine with them to form insoluble carbides. g %g%:% f$ ziggf g figg gi gl g During the heating for the quench-temper prior treatment at conventional temperatures, or even at much higher temperatures up to say 1800 F., little if any austenite forms and the resultant quenched and tempered structure will contain large amounts of excess ferrite.

Such a structure has a very low notch-impact value after aging.

It is sometimes advantageous to nitride the surfaces of articles made from the steel of the invention. In such Table IV.Efiect of balance of carbon and titanium on cases the n n' g is carried out in conventional manner,

notch impact value f age h d d Steel and since the nitriding temperature and age hardening temperature are about the same for this steel, nitn'ding and age hardening may be eifected concurrently. The core of Table III.Efiect of balance of carbon and columbium on notch impact value of age hardened steel Analysis, Percent Bar a Ob 0 Ni Mo Al Hardness V-notch Ratio Rockwell Izod, 0" ft. lbs.

[Treatment prior to aging consisted of an oil quench from 1,650 F. and e 4 hour temper at 1,300 F. Aged for 16 hours at 900 F.}

I the resulting nitrided and age hardened article will have Analysis, Percent Hardness notch the high toughness above mentioned, namely, a minimum Bar fifig n ol on Izod, ft. notch impact value of 10 ft. lbs.

0 Ni Mo Al Ti 0 This application is a continuation-in-part of our joint parent application Serial No. 176,766, filed July 29, 1950, 2777-" .28 3.7 .70 1.3 1. 54 5.5 as 1 now abandoned. 2776-.. .23 3.7 .69 1.0 .53 1.9 39 11 We claim:

45 1. An age hardenable alloy steel characterized in being hardenable to a minimum of Rockwell C 35 on aging for about 15 hours at 900 to 1000 F., and in having It will also be seen from the data in the following table a V-HOtCh IZOd impact Value of at last tell foot Pounds that although the hardness of the steel increase with in the age hardened condition, said steel containing about: an increase in carbon content, the notchimpact value of 50, t0 6% nickel; t0 3% aluminum; P to 2% each the steel falls below 10 ft. lbs. when the carbon content f molybdenum, manganese, and chromium; p to about exceeds about 0.55% even though the ratio of the active SilicOIl; I0 055% carbon; at least one element carbide former (vanadium) is maintained within the selected from the group consisting of 0.1 to 2% vanadium, range set forth above. 0.2 to 3 columbium, 0.2 to 6% tantalum, and 0.1 to 2% Table V.Efiect of carbon on notch-impact valae. of age hardened steel [Treatment prior to aging consisted of an oil quench from 1,650 F. and a 4 hour temper at 1,300 F. Aged for 16 hours at 950 F.}

N Urn-The steels listedin Tables II, 111, and IV also contain Mn from 0.5 to 1.4%; P from .007 to 035%; S from .010 to .040; and Si from 0.2 to 0.9%; balance substantially all iron.

Analysis, Percent Ratio Hardness Bar V to C Ro cgwell ft.

0 Mn S1 Ni V Mo A1 Thus it has been shown that when the carbon and car- 70 titanium; up to 5% of other elements which do not imbide-forming elements are properly balanced, the notchpair the hardening and toughness properties of the steel;

impact value or" the steel is much higher than when an excess of the carbide forming element is present, even though the hardness of the steel may also be higher.

Without limiting ourselves to any particular explanaand the balance iron.

2. An age hardenable alloy steel characterized in being hardenable to a minimum of Rockwell C 35 on aging for about 15 hours at 900 to 1000 F., and in aluminum; 0.1 to 2% molybdenum; up to 2% each of manganese and chromium; up to 1% silicon; 0.1 to 0.55% carbon; at least one element selected from the group consisting of 0.1 to 2% vanadium, 0.2 to 3% columbium, 0.2 to 6% tantalum, and 0.1 to 2% titanium; up to 5% of other elements which do not impair the hardening and toughness properties of the steel; and the balance iron.

14. An age hardened, steel die-block for die casting, said die block having a hardness of at least Rockwell C 35 and a V-notch Izod impact value of at least ten foot pounds, said steel containing about: 2.5 to 6% nickel; 0.8 to 3% aluminum; 0.1 to 2% molybdenum; up to 2% each of manganese and chromium; up to 1% silicon; 0.1 to 0.55 carbon; at least one element selected from the group consisting of 0.1 to 2% vanadium, 0.2 to 3% columbium, 0.2 to 6% tantalum, and 0.1 to 2% titanium; up to 5% of other elements which do not impair the hardening and toughness properties of the steel; and the balance iron.

15. An age hardenable alloy steel characterized in being hardenable to a minimum of Rockwell C 35 on aging for about 15 hours at 900 to 1000 F., and in having a V-notch Izod impact value of at least ten-foot pounds in the age hardened condition, said steel containing about 2.5% to 6% nickel; 0.8% to 3% aluminum; 0.1% to 2% molybdenum; up to 2% each of manganese and chromium; up to 1% silicon; 0.1% to 0.55 carbon; at least one element selected from the group consisting of vanadium, columbium, tantalum, and titanium, the carbon content being so balanced in relation to the elements of said group that the vanadium content is from 0.3 to 4 times the carbon content, the columbium content is from 0.4 to 6 times the carbon content, the tantalum content is from 0.8 to 11 times the carbon content, and the titanium content is from 0.2 to 3 times the carbon content; up to 5% of other elements which do not impair the hardening and toughness properties of the steel; and the balance iron.

References Cited in the file of this patent UNITED STATES PATENTS 1,943,595 Foley Jan. 16, 1934 FOREIGN PATENTS 431,248 Great Britain July 3, 1935 

1. AN AGE HARDENABLE ALLOY STEEL CHARACTERIZED IN BEING HARDENEABLE TO A MINIMUM TO ROCKWELL "C" 35 ON AGING FOR ABOUT 15 HOURS AT 900* TO 1000* F., AND IN HAVING A V-NOTCH IZOD IMPACT VALUE OF AT LEAST TEN FOOT POUNDS IN THE AGE HARDENED CONDITION, SAID STEEL CONTAINING ABOUT: 2.5 TO 6% NICKEL; 0.8 TO 3% ALUMINUM; UP TO 2% EACH OF MOLYBDENUM, MANGANESE, AND CHROMIUM; UP TO ABOUT 1% SILICON; 0.1 TO 0.55% CARBON; AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF 0.1 TO 2% VANADIUM 0.2 TO 3% COLUMBIUM, 0.2 TO 6% TANTALUM, AND 0.1 TO 2% TITANIUM; UP TO 5% OF OTHER ELEMENTS WHICH DO NOT IMPAIR THE HARDENING AND TOUGHNESS PROPERTIES OF THE STEEL; AND THE BALANCE IRON. 